Thermal design of shell-and-tubeheat exchangers (STHEs) isdone by sophisticated computersoftware. However, a good un-
derstanding of the underlying principles
of exchanger design is needed to use this
software effectively.
This article explains the basics of ex-
changer thermal design, covering such
topics as: STHE components; classifica-
tion of STHEs according to construction
and according to service; data needed for
thermal design; tubeside design; shellside
design, including tube layout, baffling,
and shellside pressure drop; and mean
temperature difference. The basic equa-
tions for tubeside and shellside heat
transfer and pressure drop are well-
known; here we focus on the application
of these correlations for the optimum de-
sign of heat exchangers. A followup arti-
cle on advanced topics in shell-and-tube
heat exchanger design, such as allocation
of shellside and tubeside fluids, use of
multiple shells, overdesign, and fouling,
is scheduled to appear in the next issue.
Components of STHEs
It is essential for the designer to have a
good working knowledge of the mechani-
cal features of STHEs and how they in-
fluence thermal design. The principal
components of an STHE are:
• shell;
• shell cover;
• tubes;
• channel;
• channel cover;
• tubesheet;
• baffles; and
• nozzles.
Other components include tie-rods and
spacers, pass partition plates, impinge-
ment plate, longitudinal baffle, sealing
strips, supports, and foundation.
The Standards of the Tubular Ex-
changer Manufacturers Association
(TEMA) (1) describe these various com-
ponents in detail.
An STHE is divided into three parts:
the front head, the shell, and the rear
head. Figure 1 illustrates the TEMA
nomenclature for the various construction
possibilities. Exchangers are described by
the letter codes for the three sections —
for example, a BFL exchanger has a bon-
net cover, a two-pass shell with a longitu-
dinal baffle, and a fixed-tubesheet rear
head.
Classification
based on construction
Fixed tubesheet. A fixed-tubesheet
heat exchanger (Figure 2) has straight
tubes that are secured at both ends to
tubesheets welded to the shell. The con-
struction may have removable channel
covers (e.g., AEL), bonnet-type channel
covers (e.g., BEM), or integral tubesheets
(e.g., NEN).
The principal advantage of the fixed-
tubesheet construction is its low cost be-
cause of its simple construction. In fact,
the fixed tubesheet is the least expensive
construction type, as long as no expan-
sion joint is required.
Other advantages are that the tubes can
be cleaned mechanically after removal of
SHELL-AND-TUBE HEAT EXCHANGERS
CHEMICAL ENGINEERING PROGRESS • FEBRUARY 1998 ©Copyright 1997 American Institute of Chemical Engineers. All rights reserved. Copying and downloading permitted with restrictions.
Effectively Design
Shell-and-Tube
Heat Exchangers
Rajiv Mukherjee,
Engineers India Ltd.
To make the most
of exchanger
design software,
one needs to
understand STHE
classification,
exchanger
components, tube
layout, baffling,
pressure drop, and
mean temperature
difference.
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CHEMICAL ENGINEERING PROGRESS • FEBRUARY 1998
SHELL-AND-TUBE HEAT EXCHANGERS
n Figure 1. TEMA designations for shell-and-tube heat exchangers.
E
F
G
H
J
K
X
One-Pass Shell
Two-Pass Shell
with Longitudinal Baffle
Split Flow
Double Split Flow
Divided Flow
Cross Flow
Kettle-Type Reboiler
A
B
Removable Channel and Cover
C
N
Bonnet (Integral Cover)
Integral With Tubesheet
Removable Cover
D
Special High-Pressure Closures
T
U
W
U-Tube Bundle
Pull-Through Floating Head
Floating Head with Backing Device
S
P
N
Outside Packed Floating Head
Fixed Tube Sheet
Like "C" Stationary Head
Fixed Tube Sheet
Like "B" Stationary Head
Externally Sealed
Floating Tubesheet
Fixed Tube Sheet
Like "A" Stationary Head
Stationary Head Types Shell Types Rear Head Types
M
L
Channel Integral With Tubesheet
and Removable Cover
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the channel cover or bonnet, and that
leakage of the shellside fluid is mini-
mized since there are no flanged joints.
A disadvantage of this design is
that since the bundle is fixed to the
shell and cannot be removed, the out-
sides of the tubes cannot be cleaned
mechanically. Thus, its application is
limited to clean services on the shell-
side. However, if a satisfactory chem-
ical cleaning program can be em-
ployed, fixed-tubesheet construction
may be selected for fouling services
on the shellside.
In the event of a large differential
temperature between the tubes and
the shell, the tubesheets will be un-
able to absorb the differential stress,
thereby making it necessary to incor-
porate an expansion joint. This takes
away the advantage of low cost to a
significant extent.
U-tube. As the name implies, the
tubes of a U-tube heat exchanger
(Figure 3) are bent in the shape of a
U. There is only one tubesheet in a U-
tube heat exchanger. However, the
lower cost for the single tubesheet is
offset by the additional costs incurred
for the bending of the tubes and the
somewhat larger shell diameter (due
to the minimum U-bend radius), mak-
ing the cost of a U-tube heat ex-
changer comparable to that of a fixed-
tubesheet exchanger.
The advantage of a U-tube heat
exchanger is that because one end is
free, the bundle can expand or con-
tract in response to stress differen-
tials. In addition, the outsides of the
tubes can be cleaned, as the tube bun-
dle can be removed.
The disadvantage of the U-tube
construction is that the insides of the
tubes cannot be cleaned effectively,
since the U-bends would require flex-
ible-end drill shafts for cleaning.
Thus, U-tube heat exchangers should
not be used for services with a dirty
fluid inside tubes.
Floating head. The floating-head
heat exchanger is the most versatile
type of STHE, and also the costliest.
In this design, one tubesheet is fixed
relative to the shell, and the other is
free to “float” within the shell. This
permits free expansion of the tube
bundle, as well as cleaning of both
the insides and outsides of the tubes.
Thus, floating-head SHTEs can be
used for services where both the
shellside and the tubeside fluids are
dirty — making this the standard con-
struction type used in dirty services,
such as in petroleum refineries.
There are various types of float-
ing-head construction. The two most
common are the pull-through with
backing device (TEMA S) and pull-
through (TEMA T) designs.
The TEMA S design (Figure 4) is
the most common configuration in
the chemical process industries (CPI).
The floating-head cover is secured
against the floating tubesheet by bolt-
ing it to an ingenious split backing
ring. This floating-head closure is lo-
cated beyond the end of the shell and
contained by a shell cover of a larger
diameter. To dismantle the heat ex-
changer, the shell cover is removed
first, then the split backing ring, and
then the floating-head cover, after
which the tube bundle can be re-
moved from the stationary end.
In the TEMA T construction (Fig-
ure 5), the entire tube bundle, includ-
ing the floating-head assembly, can
be removed from the stationary end,
since the shell diameter is larger than
the floating-head flange. The floating-
head cover is bolted directly to the
floating tubesheet so that a split back-
ing ring is not required.
The advantage of this construction
is that the tube bundle may be re-
moved from the shell without remov-
ing either the shell or the floating-
head cover, thus reducing mainte-
nance time. This design is particular-
ly suited to kettle reboilers having a
dirty heating medium where U-tubes
cannot be employed. Due to the en-
larged shell, this construction has the
highest cost of all exchanger types.
FEBRUARY 1998 • CHEMICAL ENGINEERING PROGRESS
Support
Bracket
Stationary
Tubesheet
Stationary
Tubesheet
Bonnet
(Stationary
Head)
Bonnet
(Stationary
Head)
Baffles Tie Rods
and Spacers
n Figure 2. Fixed-tubesheet heat exchanger.
Tubeplate Shell Tubes BafflesHeader
n Figure 3. U-tube heat exchanger.
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There are also two types of packed
floating-head construction — outside-
packed stuffing-box (TEMA P) and
outside-packed lantern ring (TEMA
W) (see Figure 1). However, since
they are prone to leakage, their use is
limited to services with shellside flu-
ids that are nonhazardous and non-
toxic and that have moderate pres-
sures and temperatures (40 kg/cm2
and 300°C).
Classification
based on service
Basically, a service may be single-
phase (such as the cooling or heating
of a liquid or gas) or two-phase (such
as condensing or vaporizing). Since
there are two sides to an STHE, this
can lead to several combinations of
services.
Broadly, services can be classified
as follows:
• single-phase (both shellside and
tubeside);
• condensing (one side condens-
ing and the other single-phase);
• vaporizing (one side vaporizing
and the other side single-phase); and
• condensing/vaporizing (one side
condensing and the other side
vaporizing).
The following nomenclature is
usually used:
Heat exchanger: both sides single-
phase and process streams (that is,
not a utility).
Cooler: one stream a process fluid
and the other cooling water or air.
Heater: one stream a process fluid
and the other a hot utility, such as
steam or hot oil.
Condenser: one stream a condens-
ing vapor and the other cooling water
or air.
Chiller: one stream a process
fluid being condensed at sub-atmo-
spheric temperatures and the other a
boiling refrigerant or process stream.
Reboiler: one stream a bottoms
stream from a distillation column and
the other a hot utility (steam or hot
oil) or a process stream.
This article will focus specifically
on single-phase applications.
Design data
Before discussing actual thermal
design, let us look at the data that
must be furnished by the process li-
censor before design can begin:
1. flow rates of both streams.
2. inlet and outlet temperatures of
both streams.
3. operating pressure of both
streams. This is required for gases,
especially if the gas density is not
furnished; it is not really necessary
for liquids, as their properties do not
vary with pressure.
4. allowable pressure drop for
both streams. This is a very important
parameter for heat exchanger design.
Generally, for liquids, a value of
0.5–0.7 kg/cm2 is permitted per shell.
A higher pressure drop is usually war-
ranted for viscous liquids, especially
in the tubeside. For gases, the allowed
value is generally 0.05–0.2 kg/cm2,
with 0.1 kg/cm2 being typical.
5. fouling resistance for both
streams. If this is not furnished, the
designer should adopt values speci-
fied in the TEMA standards or based
on past experience.
6. physical properties of both
streams. These include viscosity,
thermal conductivity, density, and
specific heat, preferably at both inlet
and outlet temperatures. Viscosity
data must be supplied at inlet and
outlet temperatures, especially for
liquids, since the variation with tem-
perature may be considerable and is
irregular (neither linear nor log-log).
7. heat duty. The duty specified
should be consistent for both the
shellside and the tubeside.
8. type of heat exchanger. If not
furnished, the designer can choose
this based upon the characteristics of
the various types of construction de-
scribed earlier. In fact, the designer is
normally in a better position than the
process engineer to do this.
9. line sizes. It is desirable to
match nozzle sizes with line sizes to
avoid expanders or reducers. Howev-
er, sizing criteria for nozzles are usu-
ally more stringent than for lines, es-
pecially for the shellside inlet. Conse-
quently, nozzle sizes must sometimes
be one size (or even more in excep-
tional circumstances) larger than the
corresponding line sizes, especially
for small lines.
10. preferred tube size. Tube size
is designated as O.D. · thickness ·
length. Some plant owners have a
preferred O.D. · thickness (usually
based upon inventory considerations),
and the available plot area will deter-
mine the maximum tube length.
Many plant owners prefer to stan-
dardize all three dimensions, again
based upon inventory considerations.
11. maximum shell diameter. This
is based upon tube-bundle removal re-
quirements and is limited by crane ca-
pacities. Such limitations apply only to
exchangers with removable tube bun-
dles, namely U-tube and floating-head.
For fixed-tubesheet exchangers, the
only limitation is the manufacturer’s
fabrication capability and the avail-
ability of components such as dished
ends and flanges. Thus, floating-head
heat exchangers are often limited to a
shell I.D. of 1.4–1.5 m and a tube
length of 6 m or 9 m, whereas fixed-
tubesheet heat exchangers can have
shells as large as 3 m and tubes
lengths up to 12 m or more.
12. materials of construction. If
the tubes and shell are made of iden-
tical materials, all components should
be of this material. Thus, only the
shell and tube materials of construc-
tion need to be specified. However, if
the shell and tubes are of different
metallurgy, the materials of all princi-
pal components should be specified
to avoid any ambiguity. The principal
components are shell (and shell
cover), tubes, channel (and channel
cover), tubesheets, and baffles.
Tubesheets may be lined or clad.
13. special considerations. These
include cycling, upset conditions, al-
ternative operating scenarios, and
whether operation is continuous or
intermittent.
Tubeside design
Tubeside calculations are quite
straightforward, since tubeside flow
CHEMICAL ENGINEERING PROGRESS • FEBRUARY 1998
SHELL-AND-TUBE HEAT EXCHANGERS
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represents a simple case of flow
through a circular conduit. Heat-trans-
fer coefficient and pressure drop both
vary with tubeside velocity, the latter
more strongly so. A good design will
make the best use of the allowable
pressure drop, as this will yield the
highest heat-transfer coefficient.
If all the tubeside fluid were to
flow through all the tubes (one tube
pass), it would lead to a certain veloc-
ity. Usually, this velocity is unaccept-
ably low and therefore has to be in-
creased. By incorporating pass parti-
tion plates (with appropriate gasket-
ing) in the channels, the tubeside fluid
is made to flow several times through
a fraction of the total number of tubes.
Thus, in a heat exchanger with 200
tubes and two passes, the fluid flows
through 100 tubes at a time, and the
velocity will be twice what it would
be if there were only one pass. The
number of tube passes is usually one,
two, four, six, eight, and so on.
Heat-transfer coefficient
The tubeside heat-transfer coeffi-
cient is a function of the Reynolds
number, the Prandtl number, and
the tube diameter. These can be bro-
ken down into the following funda-
mental parameters: physical
properties (namely viscosity, ther-
mal conductivity, and specific heat);
tube diameter; and, very important-
ly, mass velocity.
The variation in liquid viscosity is
quite considerable; so, this physical
property has the most dramatic effect
on heat-transfer coefficient.
The fundamental equation for tur-
bulent heat-transfer inside tubes is:
Nu = 0.027 (Re)0.8 (Pr)0.33 (1a)
or
(hD/k) =
0.027 (DG/m )0.8 (cm /k)0.33 (1b)
Rearranging:
h = 0.027(DG/m )0.8(cm /k)0.33(k/D) (1c)
Viscosity influences the heat-trans-
fer coefficient in two opposing ways
— as a parameter of the Reynolds
number, and as a parameter of Prandtl
number. Thus, from Eq. 1c:
h a (m )0.33–0.8 (2a)
h a (m )–0.47 (2b)
In other words, the heat-transfer
coefficient is inversely proportional
to viscosity to the 0.47 power. Simi-
larly, the heat-transfer coefficient is
directly proportional to thermal con-
ductivity to the 0.67 power.
These two facts lead to some inter-
esting generalities about heat transfer.
A high thermal conductivity promotes
a high heat-transfer coefficient. Thus,
cooling water (thermal conductivity
of around 0.55 kcal/h•m•°C) has an
extremely high heat-transfer coeffi-
cient of typically 6,000 kcal/h•m2•°C,
followed by hydrocarbon liquids
(thermal conductivity between 0.08
and 0.12 kcal/h•m•°C) at 250–1,300
kcal/h•m2•°C, and then hydrocarbon
gases (thermal conductivity between
0.02 and 0.03 kcal/h•m•°C) at
50–500 kcal/h•m2•°C.
Hydrogen is an unusual gas, be-
cause it has an exceptionally high
thermal conductivity (greater than
that of hydrocarbon liquids). Thus,
its heat-transfer coefficient is to-
ward the upper limit of the range
for hydrocarbon liquids.
The range of heat-transfer coeffi-
cients for hydrocarbon liquids is
FEBRUARY 1998 • CHEMICAL ENGINEERING PROGRESS
Stationary-Head
Channel
Stationary
Tubesheet Shell
Support
Saddles
Floating
Tubesheet
Floating-Head
Cover
Shell
Cover
Tie Rods
and Spacers
Pass
Partition
Baffles
n Figure 4. Pull-through floating-head exchanger with backing device (TEMA S).
Shell
Weir
Support
SaddleBaffles
Support
Saddle
Floating
Tubesheet
Floating-Head
Cover
Shell
CoverStationary-Head
Channel
Tie Rods
and Spacers
Pass
Partition
n Figure 5. Pull-through floating-head exchanger (TEMA T).
rather large due to the large variation
in their viscosity, from less than 0.1
cP for ethylene and propylene to more
than 1,000 cP or more for bitumen.
The large variation in the heat-transfer
coefficients of hydrocarbon gases is
attributable to the large variation in
operating pressure. As operating pres-
sure rises, gas density increases. Pres-
sure drop is directly proportional to
the square of mass velocity and in-
versely proportional to density. There-
fore, for the same pressure drop, a
higher mass velocity can be main-
tained when the density is higher. This
larger mass velocity translates into a
higher heat-transfer coefficient.
Pressure drop
Mass velocity strongly influences
the heat-transfer coefficient. For tur-
bulent flow, the tubeside heat-transfer
coefficient varies to the 0.8 power of
tubeside mass velocity, whereas tube-
side pressure drop varies to the square
of mass velocity. Thus, with increas-
ing mass velocity, pressure drop in-
creases more rapidly than does the
heat-transfer coefficient. Consequent-
ly, there will be an optimum mass ve-
locity above which it will be wasteful
to increase mass velocity further.
Furthermore, very high velocities
lead to erosion. However, the pres-
sure drop limitation usually becomes
controlling long before erosive veloc-
ities are attained. The minimum rec-
ommended liquid velocity inside
tubes is 1.0 m/s, while the maximum
is 2.5–3.0 m/s.
Pressure drop is proportional to
the square of velocity and the total
length of travel. Thus, when the num-
ber of tube passes is increased for a
given number of tubes and a given
tubeside flow rate, the pressure drop
rises to the cube of this increase. In
actual practice, the rise is somewhat
less because of lower friction factors
at higher Reynolds numbers, so the
exponent should be approximately
2.8 instead of 3.
Tubeside pressure drop rises steeply
with an increase in the number of tube
passes. Consequently, it often happens
that for a given number of tubes and
two passes, the pressure drop is much
lower than the allowable value, but
with four passes it exceeds the allow-
able pressure drop. If in such circum-
stances a standard tube has to be em-
ployed, the designer may be forced to
accept a rather low velocity. However,
if the tube diameter and length may be
varied, the allowable pressure drop can
be better utilized and a higher tubeside
velocity realized.
The following tube diameters are
usually used in the CPI: w, 1, e, 5,
1, 14, and 11 in. Of these, 5 in. and
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